Heat-expandable microspheres comprising a structure of a shell of thermoplastic resin and a blowing agent encapsulated therein are generally called heat-expandable microcapsules. Vinylidene chloride copolymers, acrylonitrile copolymers, and acrylic copolymers are usually employed as the thermoplastic resin, and hydrocarbons, such as isobutane and isopentane, are mostly employed as the blowing agent. (Refer to Patent Literature 1.)
Such heat-expandable microcapsules are processed into lightweight hollow particles (heat-expanded microspheres) with heating and expanding. A process for spraying a dispersion of heat-expandable microcapsules in hot gas to expand and dry the microcapsules simultaneously has been proposed as a process for expanding heat-expandable microcapsules. (Refer to Patent Literature 2.) The process, however, has a problem, in that the deposition of aggregated microcapsules at the end of the spray used is highly likely.
A process for producing hollow particulates by expanding heat-expandable microcapsules, which have been produced by impregnating the same with a blowing agent, in hot gas from an electric hot gas generator has been proposed. (Refer to Patent Literature 3.) As described in comparative example 3 in Patent Literature 3, the process completely failed to produce particulates having preferable properties, when the process was applied to expand heat-expanding microcapsules in which a blowing agent was encapsulated, because the process could not control the retention time of each heat-expandable microsphere in the device, and the resultant hollow particulates had low expansion coefficients and shrunk.
Further, a process for producing porous particulates wherein dried heat-expandable microspheres on a belt conveyer are heated and expanded at a temperature higher than their expanding temperature and transferred from the conveyer with air flow has been proposed. (Refer to Patent Literature 4.) The process has a problem, in that non-uniform product is possibly produced due to varied true specific gravity and increased coefficient of variation in particle size distribution of resultant porous particulates, both of which result from unexpanded heat-expandable microspheres migrating into expanded porous particulates being transferred with air flow.
It is preferable to minimize the generation of aggregated microspheres, usually by decreasing the quantity of remaining raw material, heat-expandable microcapsules, in the expanding operation of the heat-expandable microcapsules. In order to meet such requirement, expanding each of heat-expandable microcapsules with the same heat history is considered essential.
For example, a process for expanding microcapsules wherein heat-expandable microcapsules are expanded in an aqueous slurry, and the slurry containing expanded particles is passed between grinding stones to disperse aggregated particulates, has been proposed. (Refer to Patent Literature 5.) Although the process can apply the same heat history to all of heat-expandable microcapsules, the fusion of expanded microcapsules appears as a problem when the expanded microcapsules need to be dried for blending in plastics, paints, rubber, and sealants, being different from employing the aqueous slurry.
A process for producing porous particulates wherein heat-expandable microspheres are heated and expanded after being dispersed into a slurry has been proposed. (Refer to Patent Literature 6.) The process can also apply the same heat history to all heat-expandable microspheres. But the process has problems, in that there is a low production efficiency due to the steps of deliquoring slurry containing expanded heat-expandable microspheres and drying, which are required for producing dried hollow particulates, and possible coagulation of hollow particulates, especially those produced from heat-expandable microspheres having shells with low softening points, at the drying step.
Heat-expandable microspheres, which have particle sizes distributed in a sharp peak, and their production process, have been proposed. (Refer to Patent Literature 7.) The literature discloses that the heat-expandable microspheres produced in the process expand sharply into expanded microspheres of uniform shape and size, though the expanding procedure is not clearly specified. Although the heat-expandable microspheres have uniform particle size distribution, it is apparent to those skilled in the art that expanded microspheres of uniform particle size distribution without aggregated microspheres cannot be produced in the known conventional expanding processes.
As clearly described above, it is required to minimize remaining raw material and the generation of aggregated microspheres in both dry and wet expanding processes, though it has not been attained at present.
Hollow particulates are blended in porous molded products in order to lighten the products as described above. Porous molded products are usually produced by mixing and kneading a base component, filler, and hollow particulates to prepare a porous material composition, and by molding the composition into a prescribed form. In the mixing and kneading process, hollow particulates are often subjected to great external force which damages a portion of the particulates, and it leads to a problem, which is the unattainable lightening of a product to a prescribed level. In addition, thermoplastic resin forming the shell of hollow particulates is softened with heat and pressure applied to the porous material composition in the molding process, and the hollow particulates are subjected to external pressure. The internal pressure generated by the vapor of the blowing agent encapsulated in the hollow particulates cannot resist the external pressure, and thus the hollow particulates shrink to some extent in the molding process. As a result, there are problems with the hardening and shrinking of products, such as shrunk porous molded products, poor dimensional stability, and unattainable lightening of a product to a prescribed level.
For solving those problems, a porous material composition in which heat-expandable microspheres and hollow particulates are blended has been proposed. (Refer to Patent Literature 8.) Although the hardening and shrinking are solved with the composition, it requires a complex step of preparing two different particles (heat-expandable microspheres and hollow particulates). In addition, porous molded products are thermally deflated with time, resulting in the decrease of their volume when used at high temperature, though such deflation is not remarkable when they are used at normal temperature. The cause of the deflation is considered to be the time-dependent leakage of a blowing agent encapsulated in hollow particulates.
For solving the problem, thermoplastic resins of low gas-permeability, such as acrylonitrile copolymer, are employed as the thermoplastic resin for forming the shell, though it is not enough to solve the problem.
Lightweight hollow particulates produced by heating and expanding heat-expandable microcapsules have been applied as lightening agents for resins and ceramics as mentioned above, heat insulators for thermosensitive paper and paints, bulkiness-imparting agents for nonwoven fabrics, shock-absorbers for automobile exteriors, and surface modifiers for attaining roughness on wall papers. The inventors of the present invention have recently developed novel heat-expandable microcapsules in which a specific fluorine compound is encapsulated as a blowing agent, and have found the application of porous particles produced by heating and expanding the microcapsules as a volume-retaining agent for pressure vessels. (Refer to Patent Literature 9.)    Patent Literature 1: U.S. Pat. No. 3,915,972    Patent Literature 2: Published Examined Japanese Patent Application, Sho 59-53290    Patent Literature 3: Published Unexamined Japanese Patent Application, Hei 8-217905    Patent Literature 4: Published Examined Japanese Patent Application, Hei 8-29245    Patent Literature 5: Published Unexamined Japanese Patent Application, Sho 62-201231    Patent Literature 6: Japanese Patent 2927933    Patent Literature 7: WO 99/37706    Patent Literature 8: Japanese Patent 3067932    Patent Literature 9: WO 2004/074396